I have recently stumbled upon a power-related mistake that I made which may be educational. The basic setup is that I designed an analog amplifier with some digital controls and did not power the digital circuitry correctly which then resulted in some very infrequent errors. The power supply for my amplifier was +/- 5V and a ground reference. This was just fine for the analog circuitry, sales however, therapy some digital potentiometers needed a single +5V supply to operate. Not wanting to contaminate the reference ground with current from the digital components, I decided to create a second “power” ground for the circuit. I did this by placing a 7805 +5V linear regulator between the +5V and -5V power rails and called the new position the power ground. I also placed some LEDs in series with resistors between the power rails and the power ground. Finally, I hooked up the digital electronics between the +5V rail and the newly created power ground.
At first this may seem to be a reasonable idea since the 7805 regulator should provide a voltage that was close to the ground reference. The problem is that the linea regulator essentially acts as a serial impedance between the power rail and the load (see On Semi’s linear regulator guide or the one from National). To be more specific, the 7805 in the previously described configuration acts as a variable impedance between the +5V power rail and power ground which varies to ensure that the voltage between the power ground and -5V would be maintained at 5 volts. So if the load (digital circuitry) is between the +5V rail and the power ground, the regulator cannot really do its job since it cannot drive current between its output and the -5V (its ground) rail. The right way to solve this specific problem would have been to use a 7905 negative 5 volt regulator which would provide the same approximate voltage for power ground and would have no problems driving the current to the -5V rail. The reason the circuit worked most of the time but failed sporadically was because of the LEDs between +5V, power ground and -5V. The amount of current that the digital components typically required was small compared to the LEDs current and therefore it was easily sourced, by the LED, not the regulator.
( hb206-d.pdf )